1. Field of the Invention
The present invention relates to a laser diode emitting laser light in the lamination direction.
2. Description of the Related Art
A Vertical Cavity Surface Emitting Laser (VCSEL) consumes lower power than that of an edge emitting laser diode, and is able to be directly modulated. Thus, in recent years, the VCSEL has been used as an inexpensive optical communication light source.
The VCSEL is generally provided with a columnar mesa in which a lower DBR layer, a lower spacer layer, an active layer, an upper spacer layer, an upper DBR layer, and a contact layer are layered in this order on a substrate. In one of the lower DBR layer and the upper DBR layer, to improve efficiency of current injection into the active layer and lower the threshold current, a current narrowing layer having a structure that a current injection region is narrowed is provided. Electrodes are respectively provided on the top face of the mesa and the rear face of the substrate. In the laser diode, a current injected from the electrode is narrowed by the current narrowing layer, and then injected into the active layer where light is emitted due to electron-hole recombination. The light is reflected by the lower DBR layer and the upper DBR layer, laser oscillation is generated in a given wavelength, and the light is emitted as laser light from the top face of the mesa.
In the foregoing VCSEL, instability around the current narrowing layer may lower the reliability. For example, after the current narrowing layer is formed by performing oxidation process in the course of manufacture, alloying (thermal treatment) is performed to obtain ohmic contact between the electrode and the semiconductor. At this time, in some cases, the current narrowing layer is separated. Such separation of the current narrowing layer may result from strain generated by the oxidation treatment and an oxidized species (for example, oxygen and moisture) remaining in the current narrowing layer.
To address the foregoing disadvantage, in the past, various resolution methods have been proposed. For example, in Japanese Unexamined Patent Application Publication No. 2004-179640, the following method is described. In the method, a Group semiconductor layer containing In is provided adjacent to the top and the bottom of a current narrowing layer. Thereby, entire strain is compensated.